Use bitvec_set_*() directly without external write pointer tracking to
simplify the code. This is part of IA Rest Octets (3GPP TS 44.018
§10.5.2.16) which is the last part of the message so it should not
interfere with the rest of encoding functions.
That's partially based on reverted commit 529ce885450946d85d1920fb3d1a994c3efe5849.
Change-Id: I143b3dd02aa54b9ce206d9e780a5554f6d9fd118
Related: OS#3014